Given that a maxed out IMAX 27 approaches $4700.00 (upgraded processor, upgraded GPU, 32GB RAM, 2TBSSD, etc, what do you gain/lose by stepping up to the iMac Pro similarly equipped?
It's only ~ $300.00 more.
Base iMac Pro only comes with 1TB SSD, so take ~$800 off your comparison price.
32GB
extra RAM (total 40GB) from Crucial will cost you $260 vs. $600 for 32GB
total from Apple (for shame, guys!) - total no-brainer.
So, I'm getting $3099 + $260 for RAM = $3359, i.e. $1640 difference. I think that's above most people's "only" threshold.
The question is, what are you going to be using it for? The iMac Pro's strengths are only likely to materialise if you're running multi-core and/or Open CL-optimised workloads, running 48-hour render jobs where you need ECC RAM, using 10Gbps Ethernet, or really need the bandwidth of 4 full-fat TB3 ports.
If you're running general purpose/mixed loads with a lot of single-threaded work and where the GPU is just for graphics then don't be surprised if the performance of the iMac Pro is a bit, well, not worth $1640, compared with the iMac.
Maybe the "Vega" will be good for gaming - maybe it won't. It depends a lot on drivers. Its probably going to be more satisfying than the old new Mac Pro with its dual-but-not-xfire GPUs.
Of course, this is all speculation given the current level of available information...